This project continued studying conformational changes in ClC-type chloride channel proteins. The ClC family of chloride-conducting ion channels is involved in a host of biological processes; these channels maintain the resting membrane potential in skeletal muscle, modulate excitability in central neurons, and are involved in the homeostasis of pH in a variety of intracellular compartments. Despite their physiological importance, the mechanisms by which these channels function are poorly understood. We are attempting to understand the functional properties of these proteins by examining several family members, including both eukaryotic and prokaryotic homologs. Currently, in collaboration with Henning Stahlberg in Switzerland, we formed 2d crystals of this protein under a series of conditions to reveal the structural changes underlying this conformational change. In the past year we have found a stable, low pH form of the 2d crystal which reveals a different conformation of the protein than those previously observed. We now have a 3 dimensional map of this crystal form and are completing its analysis. This is the first direct evidence of a conformational change that is part of the CLC transport process. We are currently focusing our mechanistic efforts on the lysosomal CLC, ClC-7. We have used a recently published method to retarget ClC-7 from lysosomes to the plasma membrane and are studying its transport properties using electrophysiology. Our initial characterization, nearly ready for publication, examines the external pH dependence of the ClC-7 transporter and probes the relationship between the transport cycle in a novel form of gating observed in the CLC's.